1. Field of the Invention
The present invention relates to a process for producing gas hydrate pellets, wherein a gas hydrate is first formed by reacting raw gas with raw water under predetermined temperature and pressure conditions, and subsequently shaping the gas hydrate into pellets by means of a pelletizer.
2. Description of the Related Art
In the past, proposals have been made wherein gas hydrate powder is first shaped into pellets by means of a pelletizer, and subsequently this pelletized gas hydrate is stored in a storage tank on land or in the hold of a ship (see Japanese patent application Kokai publication No. 2002-220353, for example).
Meanwhile, a continuous process for producing gas hydrate pellets as shown in FIG. 8 has also been conceived. In this process, raw gas (g) at high pressure (5.4 MPa, for example) and raw water (w) at a set temperature (4° C., for example) are fed into a first generator 1 to generate gas hydrate slurry (gas hydrate concentration: 20 wt %). The gas hydrate slurry is then physically dehydrated using a dehydrating machine 2 (gas hydrate concentration: 70 wt %). Subsequently, the dehydrated gas hydrate is fed into a second generator 3 and again reacted with raw gas (g) and hydrated/dehydrated (gas hydrate concentration: 90 wt %). Additionally, this powdered gas hydrate (a) is then cooled to a sub-zero temperature (−20° C., for example) by means of a refrigerating machine 4, thereby causing the gas hydrate to exhibit self-preservation at atmospheric pressure. In order to store the gas hydrate at atmospheric pressure, the gas hydrate is then depressurized from the gas hydrate formation pressure (5.4 MPa) to atmospheric pressure (0.1 MPa) by means of a depressurizing device 5. Subsequently, the gas hydrate is machined into pellets (p) by means of a pelletizer 6.
However, in order to store the gas hydrate at atmospheric pressure, the gas hydrate is cooled to a sub-zero temperature (−20° C., for example) by means of the refrigerating machine 4, dry powder of gas hydrate (a) is then depressurized from the pressure conditions maintained by the refrigerating machine 4 (5.4 MPa) to atmospheric pressure (0.1 MPa). If the powdered gas hydrate (a) is shaped into pellets (p) by means of the pelletizer 6 after conducting the above, there is a problem in that the gas hydrate concentration decreases by 15 wt % to 30 wt %.
In other words, the powdered gas hydrate (a), having been cooled to a sub-zero temperature (−20° C., for example) by means of the refrigerating machine 4, exists in a formation region X; more specifically, the gas hydrate (a) is subject to the conditions labeled A in FIG. 7 (5.4 MPa, −20° C. (257 K)). However, if the gas hydrate (a) is depressurized to atmospheric pressure, the gas hydrate (a) enters an unstable decomposition region Y; more specifically, the gas hydrate (a) becomes subject to the conditions labeled B in FIG. 7 (0.1 MPa, −20° C. (257 K)). Normally, gas hydrate in such a state exhibits self-preservation and the gas decomposition amount decreases. However, the gas decomposition does occur in the decomposition region until self-preservation is exhibited, and thus the decomposition amount is increased. In particular, the decomposition amount for powdered gas hydrate having a small grain size is significantly increased, due to the large specific surface area of such gas hydrate.
In addition, it has been found that if the pellet formation pressure in the pelletizer is increased, gas hydrate grains fracture and the gas decomposition amount increases. If the formation pressure is then suppressed as a result, gaps (e) occur in a pellet (p) between particles of the gas hydrate (a), as shown in FIG. 9. As a result, the specific surface area related to pellet decomposition becomes larger, and the decomposition amount is large even after pelletizing.
On the other hand, gas hydrate having a small grain size is strongly adhesive, and may cause blockage in the depressurizing device 5 or its surrounding pipes. As a result, there is a problem in that pellets can no longer be continuously produced.